Cilia are important motile cell organelles of organisms including man. This study continues a comprehensive attack on the structure and function of cilia, centering on ultrastructural correlates of ciliary motion, including analysis of the mechanism of movement and its regulation. This work has been influential in the development of the sliding microtubule model of ciliary motility, now widely accepted. Despite the success of this model, fundamental questions remain regarding the basic interaction responsible for sliding and the hierarchy of regulatory precesses between the basic sliding event and actual ciliary motility. This study seeks to elucidate these questions (1) by continued examination of the electron microscopy of sliding, involving particularly analysis of dynein arm structure and (2) characterization of ciliary arrest by inhibitors including high calcium and vanadate ion concentrations. This information should lead to further understanding of mechanisms of normal ciliary activity and ciliary malfunction in respiratory disease.